1. Field of the Invention
This invention relates to thin films used for fabricating bulk wave and surface acoustic wave transducers and more particularly to thin films of polycrystalline material or crystallites each having a crystalline orientation oriented in a preferred direction.
2. Description of the Prior Art
Bulk and surface acoustic wave transducers are fabricated with thin layers of piezoelectric material such as zinc oxide (ZnO). The transducers are used, for example, to fabricate delay lines when the delay medium is non-piezoelectric. For high frequencies greater than 100 MHz the piezoelectric material such as zinc oxide is usually sputtered onto the substrate or medium for propagation. In most bulk wave devices and in some surface acoustic wave devices the substrate or propagation medium is first coated with a thin metal layer such as gold which serves as one electrode of the piezoelectric transducer. The piezoelectric material is sputtered onto the gold and then a second metal film such as gold is deposited on top of the piezoelectric material to form the second electrode.
In order for the sputtered piezoelectric material such as zinc oxide to be useful as a piezoelectric transducer, the piezoelectric material or zinc oxide layer must have a high degree of crystalline C-axis orientation normal to the plane of the layer. The sputtered piezoelectric material may comprise many crystallites, but each crystallite should have its C-axis oriented normal to the plane of the layer. The other axes of the piezoelectric material, such as in the basal plane of a hexagonal crystalline material such as zinc oxide, does not have to be aligned with respect to other crystallites for good transduction to take place, so long as the C-axis of the hexagonal crystalline material is aligned normal to the layer of the piezoelectric material.
It is known that zinc oxide will have good crystalline C-axis orientation if it is sputtered onto thermally evaporated gold which itself has good &lt;111&gt; axis orientation normal to the metal layer. In other words, in the prior art, good piezoelectric transducers could be obtained by using a bottom electrode of gold having its &lt;111&gt; axis oriented normal to the gold layer followed by a zinc oxide layer having its crystalline C-axis of the various crystallites oriented normal to the plane of the layer or close thereto, as measured by reflection electron diffraction patterns. A top layer of gold forms the top of electrode of the transducer.
It is known that thermally evaporated gold will have its &lt;111&gt; axis oriented normal to the gold layer on the following materials: spinel, sapphire, lithium niobate, fused quartz and ordinary microscope slides.
The deposition of piezoelectric films having their C-axis perpendicular to the film is described in U.S. Pat. No. 3,655,429 which issued on Apr. 11, 1972 to John DeKlerk and assigned to the assignee herein. In U.S. Pat. No. 3,655,429 at column 12 the formation of zinc oxide films were found to have a high degree of orientation on an oriented substrate such as crystalline material. The degree of orientation was also affected by the rate of deposition and the temperature of the substrate during deposition.
U.S. Pat. No. 3,825,779 which issued on July 23, 1974 entitled "Interdigital Mosaic Thin Film Shear Transducer" by John DeKlerk describes depositing cadmium sulfide or zinc oxide on a layer of gold on a substrate of aluminum oxide (Al.sub.2 O.sub.3). A mosaic thin film shear transducer was described having the C-axis inclined at an angle of 40 degrees to the normal to the piezoelectric film layer.
U.S. Pat. No. 3,689,784 which issued on Sept. 5, 1972 entitled "Broadband, High Frequency, Thin Film Piezoelectric Transducers" by John DeKlerk and assigned to the assignee herein describes a transducer comprising a single layer of piezoelectric material such as cadmium sulfide and electrode structures of gold formed on a substrate of lithium niobate.
U.S. Pat. No. 3,632,439 which issued on Jan. 4, 1972 entitled "Method Of Forming Thin Insulating Films Particularly For Piezoelectric Transducer" by John DeKlerk and assigned to the assignee herein describes the formation of cadmium sulfide films on single crystal substrates with oriented A and C axes. The desirability of having the C-axis normal to the plane of the film is described as desirable to form high frequency piezoelectric transducer films.
U.S. Pat. No. 3,543,058 which issued on Nov. 24, 1970 entitled "Piezoelectric Transducer" by P. G. Klemens and assigned to the assignee herein describes the formation of a acoustic transducer having alternate layers of piezoelectric material wherein at least one layer has different electromechanical properties.
It is therefore desirable to form layers of gold having a predetermined orientation among its crystallites on substrate materials where heretofore only unoriented gold layers were formed.
It is further desirable to form an electrode of gold having its &lt;111&gt; axis oriented normal to its layer on films of thermally formed silicon dioxide and of sputtered silicon dioxide.
It is further desirable to form oriented layers of piezoelectric material on substrate materials where heretofore only poor orientation was obtained.
It is further desirable to form electro-acoustic transducers having superb characteristics of oriented piezoelectric material on substrate materials where heretofore only transducers having poor characteristics typical of unoriented piezoelectric material were formed.